1. Field of the Invention
The present invention relates to an image forming apparatus and a sheet heating apparatus.
2. Description of the Related Art
In the past, there have been widely known image forming apparatuses using an electrophotographic method, such as copying machines, printers, etc., and there have been commercialized a lot of image forming apparatuses which perform image formation not only in black and white but also in full color.
In addition, requests for the quality of images also become more and more sophisticated in accordance with the increasing use of electrophotographic image forming apparatuses in a variety of fields, too. Here, as one of the factors for determining the image quality, in particular the glossiness of full color images, there is pointed out the smoothness of output images.
In response to such a need, for example, Japanese patent application laid-open No. S64-35452 and Japanese patent application laid-open No. H5-216322 disclose image forming methods, respectively, in which a color toner made of a thermoplastic resin is transferred to a recording material on which a transparent resin layer made of a thermoplastic resin is formed, and then a color image is formed on the recording material by heating and melting the color toner. Additionally, in these image forming methods, a belt fixing unit is proposed as a preferable fixing method.
As such a belt fixing unit, there have been known ones that are described, for example, in Japanese patent application laid-open No H5-216580 and Japanese patent application laid-open No. H4-362679. The belt fixing units as described in these Japanese patent application laid-open Nos. H5-216580 and H4-362679 adopt a construction that a recording material carrying thereon an unfixed toner image is pressed and heated by the fixing belt composed of a heat resisting film, and the toner image is cooled to be solidified while keeping the recording material in intimate contact with the fixing belt, after which the recording material with the toner image fixed thereto is released from the fixing belt.
Accordingly, the toner image and the transparent resin layer on the surface of the recording material with the toner image being fixed while being embedded in the transparent resin layer of the recorder material are solidified along the surface configuration of the belt to provide a smooth entire surface of the recording material. As a result, the color image excellent in luster can be obtained (Japanese patent application laid-open No. 2004-151200 and Japanese patent application laid-open No. 2002-214948).
In addition, as such recording materials each having a resin layer for use with image forming apparatuses, there have been proposed a variety of recording materials, as described in Japanese patent application laid-open No. 2003-84477. In this Japanese patent application laid-open No. 2003-84477, an electrophotographic transfer sheet is proposed which is coated with a resin layer of a thickness of about 10 μm comprising, as a principle component, a thermoplastic resin having a glass-transition temperature of 85 C. or less.
A recording material with a resin layer as referred to above is very expensive in comparison with ordinary plain paper. Accordingly, in case where abnormality occurs in such an apparatus, repeating a copying operation after removal of all the recording materials in the apparatus as in the past causes a problem that leads to an increase in the running cost thereof.
In addition, in Japanese patent application laid-open No. H9-43912, Japanese patent application laid-open No. H8-272159 and Japanese patent application laid-open No. H11-327220, there have been disclosed apparatuses each including two image forming portions, i.e., a first image forming portion and a second image forming portion that is arranged at a downstream side of the first image forming portion. In particular, Japanese patent application laid-open No. H11-327220 describes that when a jam occurs at any location in either of the downstream second image forming portion and a downstream second fixing unit, a sheet material to be subsequently conveyed is switched into a second conveying path through which it is discharged to a tray that is arranged outside or inside the apparatus.